1. Field of the Invention
The present invention relates to an active noise control apparatus for reducing an in-compartment noise with a cancellation sound which is in opposite phase to the in-compartment noise.
2. Description of the Related Art
Japanese Laid-Open Patent Publication No. 6-109066 discloses an active noise control apparatus (hereinafter referred to as periodic-noise-compatible and aperiodic-noise-compatible ANCs) for reducing a periodic noise (hereinafter referred to as “engine muffled sound” or “engine noise”) caused by a vibratory noise which is produced by a vibratory noise source such as an engine or the like on a vehicle and generated periodically in the passenger compartment in synchronism with the rotation of the engine, and an aperiodic noise (hereinafter referred to as “drumming noise” or “road noise”) generated aperiodically in the passenger compartment by tire vibrations transmitted from the road through suspensions to the vehicle body when the vehicle is running.
The ANCs disclosed in Japanese Laid-Open Patent Publication No. 6-109066 include an acceleration sensor mounted on a suspension for outputting a signal based on vibrations from the road, and a plurality of microphones installed in the passenger compartment for generating respective canceling error signals based on the differences (hereinafter referred to as “canceling error sound”) between the noise in the passenger compartment and a canceling sound and outputting the generated canceling error signals to a controller. The controller generates a control signal for canceling out the noise based on a signal based on the vibrations, the canceling error signals, and an ignition pulse signal corresponding to the vibrations of the engine, and a speaker mounted in the passenger compartment outputs the canceling sound based on the control signal into the passenger compartment to reduce the noise according to a feedforward control process.
The engine noise referred to above is a periodically generated noise in a narrow frequency band having a predetermined central frequency. The periodic-noise-compatible ANC generates a control signal having a control frequency depending on the predetermined central frequency, and a speaker outputs a canceling sound having the control frequency into the passenger compartment for effectively reducing the noise in the passenger compartment.
On the other hand, the road noise is an aperiodically generated low-frequency noise having a central frequency equal to a resonant frequency of 40 [Hz], for example, determined from the resonant characteristics of the passenger compartment. The aperiodic-noise-compatible ANC is required to reduce resonant sounds at respective resonant frequencies.
If the aperiodic-noise-compatible ANC generates a control signal according to a feedforward control process, then the controller needs to comprise an FIR adaptive filter and a DSP (Digital Signal Processor) for performing convolutional calculations at the respective resonant frequencies. As a result, the aperiodic-noise-compatible ANC is relatively expensive to manufacture. Furthermore, since the aperiodic-noise-compatible ANC generates a control signal at the resonant frequencies while sequentially updating the filter coefficient of the adaptive filter, the controller suffers an increased computational burden for generating the control signal.
If the aperiodic-noise-compatible ANC generates a control signal according to a feedback control process, then the controller needs to comprise a combination of many analog filters for generating a control signal at the resonant frequencies. As a result, the controller has a large circuit scale, causing the ANC including the controller to have a large unit size. However, it is difficult to find a sufficient installation space for the ANC having such a large unit size in the vehicle. In addition, it is also difficult to combine the ANC having the large unit size with a digital audio unit.